1. Field of the Invention
The present invention relates to an electric discharge machining using a wire cut type electrode to control a work groove width (a width of cut or a machined groove width) by controlling a work current so as to improve the accuracy of the work.
2. Description of the Prior Art
As shown in FIG. 1, the conventional wire cut electric discharge machine usually uses a wire electrode (1) having a diameter of 0.05 to 0.3 mm made of copper, brass or tungsten. The electric discharge is repeated by the pulse current fed from the work power source (4) at a minute gap between the wire electrode (1) and a workpiece (2) on a table (10) moved in the X-direction and Y-direction under a flow of work liquid (3) (usually deionized water passed through an ionexchange resin).
The control of the relative movement of the wire electrode (1) and the workpiece and (2) to the work direction will be illustrated.
The average voltage between the electrodes (Eg) detected during the machining is compared to a reference voltage (Eo) by a comparator (5) by applying the voltages to the input terminals (5Tg), (5To) of the comparator (5). The analog output voltage of the comparator, which is (5) proportional to the difference between the voltages (Eg), (Eo) is applied to an A/D converter wherein the analog output voltage is converted into the corresponding digital data signal. The digital data signal is fed into a computer (7). On the basis of the input digit data signal and a move command (.DELTA.X, .DELTA.Y) given by an N/C tape (8), the computer calculates a machining feed rate ##EQU2## and speeds V.sub.x, V.sub.y for moving the X-Y cross table (10) in the X-direction and Y-direction. The signal corresponding to the calculated feed rates V.sub.x, V.sub.y is fed into a drive control device (9).
The cross table (10) is shifted in the X-direction and Y-direction by a motor (11.sub.x) for X-axis and a motor (11.sub.y) for Y-axis which are controlled by the drive control device (9). The machining feed rate is controlled so that the average voltage between electrodes (Eg) remains constant EQU (Eg-Eo.apprxeq.0).
In the conventional method, the pulse voltage of the work power source (4) (current peak value, pulse width and dwell) and the non-load voltage are not usually varied. Therefore, the average machining current (I) flowing between wire electrode (1) and workpiece (2) from the work power source (4) does not vary under normal conditions.
The inventors have studied by experiments, the machining feed rate and the work groove width etc. in the machining process when which the thickness of the workpiece (2) which is stepwise varied as shown in FIG. 2 by steps by the control method using a conventional electric discharge machine shown in FIG. 1. In FIG. 2, the reference ST0 designates a first step part having a thickness (T) of 5 mm; ST1 designates a second step part having a thickness of 10 mm; ST2 designates a third step part having a thickness of 20 mm; ST3 designates a fourth step part having a thickness of 40 mm; and ST4 designates a fifth step part having a thickness of 60 mm.
FIG. 3 is a graph showing the relation of the machining feed rate (F) and the average machining current (I) using the thickness of the workpiece as a parameter. It is understood from FIG. 3, that the machining feed rate (F) is in proportional to the average machining current (I) in all of the steps regardless of the thickness (T) of the workpiece.
FIG. 4 is a graph in logarithmic scales showing the relation of the machining feed rate (F) and the thickness (T) of the workpiece using the average machining current (I) as a parameter.
It is understood, from FIG. 4, that the feed rate (F) is linearly reduced depending upon the thickness (T) of the workpiece regardless of the average machining current (I). When an empirical formula of the machining feed rate (F) is obtained from FIG. 4, it will be as follows: EQU F=14.3(I-0.228).multidot.T.sup.-1.16 ( 1)
F: machining feed rate (mm/min)
I: average machining current (A)
T: workpiece thickness (mm)
If formula (1) is rearranged, the following is obtained. EQU I=0.228+0.07F.multidot.T.sup.1.16 ( 2)
If the machining feed rate (F) during machining and the workpiece thickness (T) can be obtained from Formula (2), the value of average machining current (I) during machining can be readily calculated.
FIG. 5 shows the relation between the work groove width (S) and workpiece thickness (T) uner the conditions in FIG. 4 using average machining current (I) as a parameter. If workpiece thickness (T) stepwise varies from 5 to 60 mm in the conventional machining method, the work groove width increases about 50 .mu.m at most as workpiece thickness (T) increases. Accordingly, as shown in FIG. 5, when the thickness of workpiece (2) increases in the order of t1.fwdarw.t2.fwdarw.t3.fwdarw.t4(t1&lt;t2&lt;t3&lt;t4), the work groove width increases S1.fwdarw.S2.fwdarw.S3.fwdarw.S4(S1&lt;S2&lt;S3&lt;S4).
The relations of t1 to t4 are given by t1&lt;t2&lt;t3&lt;t4 and the relations of S1 to S4 are given by S1&lt;S2&lt;S3&lt;S4.
FIGS. 6a and 6b show phenomena increasing the work groove width depending upon the increase of the workpiece thickness. FIG. 6a is a side view and FIG. 6b is a plane view. In FIGS. 6a and 6b, the reference (12a) designates a work groove formed by machining in the forward movement and (12b) designates a work groove formed in the returning movement.
As shown in FIG. 6b, generally, the wire electrode (1) is selected by shifting by a half of the work groove width value S1 as offset (Qf) relative to contour line (13) of the desired form as the wire path (12). (In this case, a half of work groove width S1 is used as an offset amount at thickness of t1.) As is clear from the figures, an overcut will occur at the inside of contour line (13) with an increase of thickness. In order to obtain a workpiece having thicknesses L shown in FIG. 6b, it is necessary to reciprocally move the wire electrode (1) to form a pair of the work grooves (12a), (12b). Therefore, the error in the reciprocal machining is double that of one-way machining, which deteriorates the accuracy.
In order to describe these phenomena 3-dimensionally, referring to FIG. 7a of the plane view and FIG. 7b of the side view, the machining volume when the wire electrode (1) advances from W1 to W2 in a unit time (1 minute in the experiment) will be illustrated. As is understood, the machining volume is expressed by the product of 3 factors: the work groove width (S), the forward movement (F) per unit time and the thickness (T) of the workpiece (2). FIG. 8 shows the relation of the product to the workpiece thickness (T) as plotted on the logarithmic paper using the average machining current as a parameter. According to this figure, the machining volume (S.F.T) per unit time decreases rectilinearly as the workpiece thickness increases; that is, the current efficiency drops. If an empirical formula of the work groove width (S) is obtained from FIG. 8, it will be as follows. EQU S=3.56(I-0.189).multidot.T.sup.-1.11 .multidot.F.sup.-1 ( 3)
If formula (1) is substituted into formula (3), the following will be obtained. ##EQU3## If formula (4) is rearranged, the following will be obtained. ##EQU4## Formula (5) suggests that the average machining current I, when the work groove width (S) is kept constant depending upon the workpiece thickness, can be obtained by calculation. The diameter of the wire electrode used for this experiment was 0.2 mm in all the tests. As is disclosed from the result of the experiment, with the conventional machining method the change in the work groove width when the workpiece thickness varies is considerably large as is clear from FIG. 5, which causes the work accuracy to be lower and cause a serious problem. In view of the problems described so far, the persent invention is to provide a method of controlling to give the constant work groove width regardless of the variation in workpiece thickness during the machining, by employing the empirical formula derived from the results of the experiment and thereby improve the work accuracy.